Submerged nozzle for continuous thin-slab casting

ABSTRACT

A submerged nozzle for continuous thin-slab casting by which a high quality thin-slab strand may be stably supplied for a long time in a continuous manner is provided, as a drift current and surface fluctuation of molten steel in the nozzle and a mold are prevented, and adhesion of ground metal to the nozzle, the damages of the nozzle and the like are suppressed even under long time use. In the submerged nozzle for continuous thin-slab casting, which comprises: a molten steel entrance port at the upper end; a molten steel stream channel with a shape of a tube extending downward from the molten steel entrance port; and a molten steel discharge opening at the lower end and in which the molten steel stream channel includes an upper section with a shape of a circular cylinder, an intermediate section with a transforming shape from a shape of a circular cylinder to a shape of a flat cylinder, and a lower section with a shape of a flat and straight tube, a cross sectional area at the flat and straight tube section in the lower section is configured to be equal to or larger than 95% or equal to or less than 105% of a cross sectional area at the cylindrical section in the upper section of the molten steel stream channel.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a submerged nozzle for continuous thin-slab casting and, more particularly, to a flat submerged nozzle for continuous thin-slab casting, wherein the nozzle is preferred for continuous casting of thin slab strand.

[0003] 2. Description of the Related Art

[0004] Recently, the thickness of a slab strand has become thinner and thinner, aiming at reduction in the post steps for continuous casting. Continuous casting of a slab strand with dimensions of, for example, equal to or less than 100 mm in thickness and equal to or larger than 1000 mm in width has been generalized. Along with the above generalization, a mold and a submerged nozzle have been required to have a flat shape.

[0005] However, there have been generated various kinds of problems which have never been occurred in use of a conventional simple mold with a rectangular shape when continuous casting of thin slab strand is done, using a nozzle and a mold with a flat shape.

[0006] For example, there have been risks of damages at the tip section of the nozzle with a flat shape by a solidified shell, which extraneous matters such as slugs which have grown on the outside wall section of the nozzle come into contact with and adhere to, and the breakout due to the broken solidified shell when long-time continuous casting is done under a condition in which the distance between the mold and the nozzle is small.

[0007] Moreover, it is not easy to fill molten steel into a flat stream channel of the submerged nozzle and, especially, when a cross sectional area of an molten steel entrance port is smaller than that of the molten steel stream at a section with a flat shape, an unstable molten steel stream such as a drift current, a turbulent stream, a pulsating stream, and the like is easily generated. Thereby, fluctuation of a molten steel surface and entrainment of mold powder into the cast strand have been easily caused.

[0008] Moreover, when a drift current is generated in a flat, and narrow molten steel stream channel, there have been problems that local dissolved-loss of the inner wall of the nozzle is easily generated at a section with a large stream rate of the molten steel, and the blockage of the nozzle caused by adhesion of nonmetallic inclusions such as alumina tends to occur at a section with a small stream rate of the molten steel.

[0009] On the other hand, when a cross sectional area of an molten steel entrance port is lager than that of the flat molten steel stream channel, there have been disadvantages that the fluctuation of the molten steel surface is easily generated and the local dissolved-loss at the lower section of the nozzle is caused without difficulty in the submerged nozzle with a flat shape, as the stream rate of a discharge stream of the molten steel becomes large.

[0010] In order to solve the above-described disadvantages brought about by drift currents which are caused by the nozzles with a flat shape, various kinds of nozzles with a flat shape, which have improved shapes of the molten steel stream channels, respectively, have been proposed so far.

[0011] For example, a submerged nozzle for continuous casting, which comprises an intermediate section with a shape widened toward the end with an appropriate angle to the horizontal plane and a lower section with a flat shape, has been disclosed in the Japanese Unexamined Patent Application Publication NO. 11-047897.

[0012] However, the nozzle with the above-described shape widened toward the end has had problems that deterioration in the quality of the cast strand is easily caused by entrainment of the mold powder and the local dissolved-loss on the wall surface of the nozzle is caused without difficulty in the submerged nozzle, as it is not easy to obtain a filled stream into the whole molten steel stream channel in the nozzle and the drift current is easily caused.

[0013] In addition, for example, a nozzle shown in FIG. 5A and FIG. 5B may be listed as another conventional nozzles with a flat shape. FIG. 5A is a longitudinal section seen from the short side and FIG. 5B is one seen from the long side. In the conventional nozzle with a flat shape, which is shown in FIG. 5A and FIG. 5B, a molten steel stream channel 23 has a shape of a circular cylinder at an upper section 23 a, a taper shape thickened toward the end at the long side of an intermediate section 3 b and a taper shape diminished toward the end at the short side of the intermediate section 3 b, and a flat shape at a lower section 23 c. Furthermore, molten steel discharge openings 24 are provided at both of the side walls of the short side.

[0014] Thus, in order to suppress solidification and adhesion of the molten steel (ground metal) in the flat and narrow molten steel stream channel 23, the molten steel discharge openings 24 are formed in the flat-shaped nozzle shown in FIG. 5A and FIG. 5B so that the molten steel stream is discharged in two directions.

[0015] However, for example, when a stream from a tundish to which the nozzle is connected is controlled by a stopper head, the stream velocity of the molten steel from the molten steel discharge openings 24 at starting of casting tends to become comparatively large in the flat-shaped nozzle in which the above-described molten steel discharge openings 24 for streams in the two directions are provided as it is difficult to execute precise stream control at starting time of the casting.

[0016] Moreover, since the molten steel discharge openings 24 are configured so that the molten steel stream runs against the mold sidewall, a lot of splashes are easily generated at an early period of casting. When the above splashes are solidified at the upper part of the mold, these solidified pieces remain as foreign substances at the start of drawing the cast strand. The solidified shell in the mold is broken from the relevant place to cause a risk of the breakout.

SUMMARY OF THE INVENTION

[0017] The present invention has been made to solve the above-described technical problems and the object is to provide a submerged nozzle for continuous thin-slab casting which a high quality thin-slab strand may be stably supplied, as a drift current and surface fluctuation of molten steel in the nozzle and the mold are prevented and the adhesion of the ground metal to the nozzle, the damages of the nozzle and the like are suppressed even under long time use.

[0018] The submerged nozzle for continuous thin-slab casting according to the present invention, which comprises: a molten steel entrance port at the upper end; a molten steel stream channel with a shape of a tube extending downward from the molten steel entrance port; and a molten steel discharge opening at the lower end and in which the molten steel stream channel includes an upper section with a shape of a circular cylinder, an intermediate section with a transforming shape from a shape of a circular cylinder to a shape of a flat cylinder, and a lower section with a shape of a flat and straight tube, is characterized in that a cross sectional area at the flat and straight tube section in the lower section is equal to or larger than 95% or equal to or less than 105% of a cross sectional area at the cylindrical section in the upper section of the molten steel stream channel.

[0019] Thus, the fluctuation of the molten steel surface caused in the mold and deterioration in the quality of the cast strand caused by entrainment of the mold powder due to the drift current may be prevented by a configuration in which the cross section in the vicinity of the molten steel entrance port is approximately equal to that of the molten steel stream channel in the vicinity of the discharge opening.

[0020] Another aspect of the submerged nozzle for continuous thin-slab casting according to the present invention, which comprises: a molten steel entrance port at the upper end; a molten steel stream channel with a shape of a tube extending downward from the molten steel entrance port; and a molten steel discharge opening at the lower end and in which the molten steel stream channel includes an upper section with a shape of a circular cylinder, an intermediate section with a transforming shape from a shape of a circular cylinder to a shape of a flat cylinder, and a lower section with a shape of a flat and straight tube, is characterized in that the intermediate section of the molten steel stream channel is provided with a taper shape thickened toward the end at the long side and a taper shape diminished toward the end at the short side and has a configuration in which the position of the starting point of the taper at the long side and that of the starting point of the taper at the short side, and that of the terminal point of the taper at the long side and that of the terminal point of the taper at the short side are different from each other with regard to their heights, a spill port is provided at the bottom center in the bottom wall of the nozzle, and the port area of the spill port is within the range of equal to or larger than 20% through equal to or less than 40% of the cross sectional area at the straight tube section in the lower section of the molten steel stream channel.

[0021] By provision of the above-described spill port, a side stream at an early period of casting may be suppressed and generation of the splashes-, which are caused when the initial molten steel stream with a great speed streams against the mold sidewall may be prevented.

[0022] In addition, by the above-described configuration, a high quality thin-slab strand may be stably supplied for a long time, as contact between a solidified shell in the mold and the lower end of the nozzle is prevented and an unstable molten steel stream such as the drift current is suppressed.

[0023] Even in the above-described nozzle, it is preferable in a similar manner to the above description that the cross sectional area at the flat and straight tube section in the lower section of the molten steel stream channel is equal to or larger than 95% or equal to or less than 105% of the cross sectional area at a cylindrical section in the upper section.

[0024] In addition, it is preferable that the above-described nozzle according to the present invention has a configuration in which at least a section which is submerged in molten steel is formed of a fire resistant material which does not contain silica glass.

[0025] Thereby, the nozzle may be stably used for a long time without reduction in the strength even in the case of provision of the spill port on the bottom wall of the relevant nozzle, as prevention of the adhesion of the molten steel to the nozzle, that of the dissolution loss of the nozzle, and improvement in the strength at the vicinity of the spill port may be realized.

[0026] Furthermore, it is preferable that the position of the terminal point of the taper at the long side and that of the terminal point of the taper at the short side are different from each other by equal to or larger than 10 mm with regard to their heights in the intermediate section of the molten steel stream channel.

[0027] Since the position of the terminal point of the taper at the long side and that of the terminal point of the taper at the short side are configured to be different from each other with regard to their heights by equal to or larger than 10 mm when the tapers are formed, the cracks, damages, and the like of the nozzle, which are caused by concentrations of the thermal stresses exerted on the terminal points of the relevant tapers may be eliminated,

[0028] In addition, it is preferable that the above-described molten steel discharge openings have a configuration in which it is formed between the side wall of the nozzle lower end at the short side and the side part of the plane-like bottom wall, and an angle α between the side wall of the nozzle at the lower end, which forms the above-described molten steel discharge opening, and the horizontal plane, and an angle β between the side part of the above-described bottom wall, which forms the above-described molten steel discharge opening, and the horizontal plane are as follows: 0°≦α≦60°, and 30°≦β≦80°.

[0029] Thus, the direction of the molten steel stream may be appropriately controlled, as the molten steel discharge openings are formed with the above-described range of angles like an “L”-shape between the side walls of the nozzle lower end at the short sides and the nozzle bottom wall as described above, and thereby, the drift current of the molten steel in the mold may be prevented.

[0030] Furthermore, fluctuation of the molten steel surface in the mold, mixture of the mold powder on the molten steel surface into the thin-slab strand, and the like may be evaded. In addition, nonmetallic inclusions in the molten steel are floated and removed, and thereby, the quality of the cast strand may be improved

[0031] In addition, it is preferable that the above-described molten steel discharge openings have a configuration in which a notch in the direction of the long side at the bottom wall of the nozzle is between equal to or larger than 75% and equal to or less than 200% of the thickness of the side wall at the short side in length.

[0032] By the above-described configuration of the molten steel discharge openings, the side stream and the downward stream of the molten steel in the mold may be adjusted to appropriate states, the drift current may be suppressed, and the sufficient stream quantity and the satisfactory stream rate of the molten steel may be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Hereinafter, the present invention will be explained in detail, referring to the following attached drawings.

[0034]FIG. 1A is a schematic longitudinal cross section showing one example of a submerged nozzle for continuous thin-slab casting according to the present invention, wherein the cross section is seen from a short side;

[0035]FIG. 1B is a schematic longitudinal cross section showing one example of a submerged nozzle for continuous thin-slab casting according to the present invention, wherein the cross section is seen from a long side;

[0036]FIG. 2A is an enlarged cross section of the nozzle shown in FIG. 1A and FIG. 1B, which is seen along the direction of a molten steel stream and shows a cross section at an upper section 3 a;

[0037]FIG. 2B is an enlarged cross section of the nozzle shown in FIG. 1A and FIG. 1B, which is seen along the direction of a molten steel stream and shows a cross section at a lower section 3 c;

[0038]FIG. 3 is an enlarged view of the III section in FIG. 1B;

[0039]FIG. 4 is an exemplary view expressing a use mode of the nozzle shown in FIG. 1A and FIG. 1B and a stream state of molten steel;

[0040]FIG. 5A is a schematic longitudinal cross section showing one example of a conventional submerged nozzle for continuous thin-slab casting, wherein the cross section is seen from a short side; and

[0041]FIG. 5B is a schematic longitudinal cross section showing one example of a conventional submerged nozzle for continuous thin-slab casting, wherein the cross section is seen from a long side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] As shown in FIG. 1A and FIG. 1B, a nozzle 1 according to the present invention comprises: a molten steel entrance port 2 at the upper end; a molten steel stream channel 3 with a shape of a tube extending downward from the above-described molten steel entrance port 2; and molten steel discharge openings 4 at the lower end, wherein the above-described molten steel stream channel 3 has a shape of a circular cylinder at an upper section 3 a, and a shape of a flat and straight tube at a lower section 3 c. In addition, an intermediate section 3 b of the above-described molten steel stream channel 3 is provided with a taper shape thickened toward the end at the long side and a taper shape diminished toward the end at the short side.

[0043] Here, the shape of a flat and straight tube expresses that each of nozzle cross sections at each height has a flat cross section and the widths of the long and short sides of the flat cross sections are not substantially changed, respectively. Then, it is preferred that the ratio between the inner diameter of the short side and that of the long side is between 1:2 and 1:10 in the cross section of the above-described flat and straight tube section.

[0044] Moreover, it is preferable in the flat and straight tube section that the corner sections are chamfered into curved surfaces both at the inner wall and the outside wall and, more preferable that the sections at the short side are configured to be a shape of a semicircle.

[0045] By the configuration in which the cross section of the lower section 3 c is flat and it is configured to be not widened toward the end, but to be like a shape of a straight tube as described above, thermal stresses are hardly applied there at use of the nozzle to prevent cracks, damages, and the like.

[0046] Moreover, since the lower section 3 c has a shape of a straight tube, an unstable molten steel stream such as a turbulent stream, a pulsating stream, a drift current, and the like is suppressed and breakout, which is caused by contact between a solidified shell in a mold and the lower end of the nozzle, is effectively prevented.

[0047] Furthermore, as shown in FIG. 2A and FIG. 2B the cross sections of the molten steel stream channel of the nozzle according to the present invention have a circular shape at the upper section 3 a and a flat and approximately rectangular shape at the lower section 3 c, and the short side is configured to be like a curved surface.

[0048] In addition, the present invention has a configuration in which a cross sectional area S_(3c) at the flat and straight tube section in the lower section 3 c of the molten steel stream channel is equal to or larger than 95% or equal to or less than 105% of a cross sectional area S_(3a) at a cylindrical section in the upper section 3 a, and, more preferably, in which S_(3a) is equal to S_(3c).

[0049] When the cross sectional area S_(3c) at the flat and straight tube section in the lower section 3 c of the molten steel stream channel is less than 95% of a cross sectional area S_(3a) at the cylindrical section in the upper section 3 a, fluctuation of a molten steel surface is easily caused in the mold, as the stream velocity of the discharge stream of the molten steel from the discharge opening of the nozzle.

[0050] On the other hand, when the cross sectional area S_(3c) at the flat and straight tube section in the lower section 3 c of the molten steel stream channel exceeds 105% of the cross sectional area S_(3a) at the cylindrical section in the upper section 3 c, deterioration in the quality of the obtained cast strand is easily caused by entrainment of the mold powder, as it is not easy to obtain a filled stream at the whole molten steel stream channel in the nozzle and the drift current is easily caused.

[0051] Accordingly, it is preferable that the cross sectional areas are set as S_(3a)=S_(3c), that is, the cross section in the vicinity of the molten steel entrance port is equal to that of the molten steel stream channel in the vicinity of the discharge opening. Thereby, fluctuation of a molten steel surface caused in the mold and deterioration in the quality of the cast strand caused by entrainment of mold powder due to the drift current may be prevented.

[0052] Moreover, a spill port 6 is provided at the bottom center in the bottom wall of the nozzle shown in FIG. 1A and FIG. 1B.

[0053] For example, when a stream from a tundish to which the nozzle is connected is controlled by a stopper head, the stream velocity of the molten steel at starting of casting generally becomes comparatively large as the flat molten steel stream channel 3 c of the nozzle has a straight tube shape.

[0054] Moreover, since the molten steel discharge openings 4 are configured so that the molten steel stream streams against the mold sidewall, a lot of splashes are easily generated at an early period of casting. When the splashes are solidified at the upper part of the mold, these solidified pieces remain as foreign substances at the start of drawing the cast strand. The solidified shell in the mold is broken from the relevant place to cause a risk of the breakout.

[0055] Accordingly, since the molten steel stream channel with the above-described shape of a flat and straight tube is formed at the lower section 3 c and the above-described spill port 6 is provided, a part of the discharged molten steel stream is guided to the relevant spill port 6 at an early period of casting and a side stream may be suppressed. Accordingly, generation of the splashes, which are caused when the initial molten steel stream with a great speed streams against the mold sidewall may be prevented.

[0056] It is preferable from the viewpoint of prevention of generation of an unstable molten steel stream that the port area of the above-described spill port 6 is within the range of equal to or larger than 20% through equal to or less than 40% of the cross sectional area at the straight tube section in the lower section 3 c of the molten steel stream channel. More preferably, the range is within the range of equal to or larger than 25% through equal to or less than 35%.

[0057] When the ratio of the port area of the above-described spill port 6 is less than 20%, the molten steel hardly streams out from the relevant spill port 6 and it becomes difficult to execute the above-described function as a spill port.

[0058] On the other hand, when the port area exceeds 40%, the strength of the nozzle bottom wall is reduced.

[0059] Here, it is preferable from the viewpoint of the strength that the cross sectional shape of the above-described spill port 6 is a shape without corners, for example, a circle, an ellipse, or the like.

[0060] In addition, the position of the starting point B of the taper at the long side and that of the starting point B′ of the taper at the short side, and that of the terminal point C of the taper at the long side and that of the terminal point C′ of the taper at the short side, are configured to be different from each other with regard to their heights in the intermediate section 3 b of the above-described molten steel stream channel 3.

[0061] In the nozzle according to the present invention, the concentrations of the thermal stresses exerted on the following points are suppressed during use of the nozzle and the damages of the nozzle such as cracks, breakage, and the like caused therefrom may be eliminated, as the position of the starting point B of the taper at the long side and that of the starting point B′ of the taper at the short side, and that of the terminal point C of the taper at the long side and that of the terminal point C′ of the taper at the short side, are configured to be different from each other with regard to their heights.

[0062] It is preferable from the viewpoint of eliminating the above concentrations of the thermal stresses in an effective manner that the position of the terminal point C of the taper at the long side and that of the terminal point C′ of the taper at the short side are different from each other by equal to or larger than 10 mm with regard to their heights.

[0063] However, with regard to the height of the terminal point C of the taper at the long side and that of the terminal point C′ of the taper at the short side, it does not matter which of the above heights is upper.

[0064] Moreover, it is preferable in the above-described nozzle that at least the part 5 which is submerged in the molten steel is formed of a fire resistant material which does not contain silica glass.

[0065] Conventionally, a fire resistant material which contains silica glass has been used for the part 5 which is submerged in the molten steel in order to improve the spalling resistance.

[0066] However, it has been known that the silica glass disappears at about 1400° C. under the reducing atmosphere, and the fire resistant material containing the silica glass is easily made porous as the silica glass disappears when the nozzle is used at a high temperature of 1500° C. or more. Thereby, there have been problems that the ground metal easily adheres to the nozzle as the metal is infiltrated into the generated pores and the strength is reduced due to the dissolved loss of the nozzle.

[0067] Moreover, when a spill port is provided on the bottom wall of a nozzle using the fire resistant material including the silica glass and the nozzle is used for a long time, the nozzle has been easily damaged at the vicinity of the spill port, as the relevant spill port is blockaded due to adhesion of the molten steel, or the strength is reduced by the dissolution loss.

[0068] Accordingly, it is preferable from the viewpoint of preventing the adhesion of the molten steel, which is caused by the porous state of the silica component, that a material, which does not contain silica glass, is used for the fire resistant material forming the part 5 which is submerged in the molten steel.

[0069] For example, alumina-graphite material, zirconia-graphite material, magnesia-graphite material and the like may be listed as a fire resistant material which does not contain the above-described silica glass and may be preferably used for the nozzle according to the present invention.

[0070] Moreover, the above-described fire resistant material does not contain the silica glass, but there is no problem even if the material contains equal to or less than 5% crystalline silica compound, as the blockage of the spill port, the damage at the vicinity of the port and the like are not caused. Specifically, for example, agalmatolite including pyrophyllite (Al₂O₃.4SiO₂.H₂O) as a principal element, potter's clay including kaolinite (Al₂O₃.2SiO₂.2H₂O) as a principal element, and the like may be used for a raw material of the fire resistant material forming the part 5 which is submerged in the molten steel.

[0071] By using the above-described fire resistant material which does not contain the silica glass prevention of the adhesion of the molten steel to the nozzle, that of the dissolution loss of the nozzle, and improvement in the strength at the vicinity of the spill port may be realized, and the nozzle may be stably used without the blockage of the spill port and the damage at the vicinity of the port.

[0072] Furthermore, it is also possible to increase the port area of the spill port in comparison with a conventional one.

[0073] Here, the above-described material may be applied to a fire resistant material which forms other parts except the part 5 which is submerged in the molten steel.

[0074] In addition, the above-described molten steel discharge openings 4 preferably have a configuration in which it is formed between the side wall of the nozzle lower end at the short side and the side part of the plane-like bottom wall, and an angle a between the side wall of the nozzle at the lower end, which forms the above-described molten steel discharge opening, and the horizontal plane, and an angle β between the side part of the above-described bottom wall, which forms the above-described molten steel discharge opening, and the horizontal plane are as follows: 0°≦α≦60°, and 30°≦β≦80°.

[0075] By the configuration in which the molten steel discharge openings 4 are not formed like a straight line, but are made like an “L”-shape between the side walls of the nozzle lower end at the short sides and the nozzle bottom wall as described above, a horizontal molten steel discharge stream may be obtained in the approximately horizontal direction. Then, the molten steel stream streams against the mold sidewall to generate an upstream, nonmetallic inclusions in the molten steel are floated and removed, and thereby, the quality of the cast strand may be improved.

[0076] Moreover, the drift current of the molten steel in the mold may be prevented, and fluctuation of the molten steel surface which may cause deterioration of the quality of the cast strand and mixture of the mold powder on the molten steel surface into the thin-slab strand and the like may be evaded.

[0077] In addition, the direction of the molten steel stream may be appropriately controlled as the above-described “L”-shape discharge opening 4 has a configuration in which the side wall, which forms the opening, of the nozzle at the lower end has a diminishing gradient toward the end, the angle α between the side wall and the horizontal plane is 0°≦α≦60°, the above-described bottom wall forming the above discharge opening has a thickening gradient toward the end, and the angle β between the bottom wall and the horizontal plane is 30°≦β≦80°.

[0078] Moreover, the above-described molten steel discharge openings 4 preferably have a configuration, as shown in FIG. 3, in which a notch T in the direction of the long side at the bottom wall of the nozzle is between equal to or larger than 75% and equal to or less than 200% of the thickness t of the side wall at the short side in length, that is, the notch T is formed as follows: 75%≦T/t≦200%.

[0079] When the above-described angle β is less than 30°, the drift current of the molten steel is easily generated in the mold, especially at the lower part of the nozzle under a case in which T/t is less than 75%, as the amount of the downward stream of the molten steel is little, and that of the side stream is much.

[0080] On the other hand, when the above-described angle β is equal to or larger than 80°, the molten steel is easily solidified on the molten steel surface at the vicinity of the nozzle under a case in which T/t exceeds 200%, as the amount of the side stream of the molten steel is little.

[0081] In addition, the submerged nozzle for continuous thin-slab casting according to the present invention is fitted at the bottom section of the tundish 11 and a stopper head 12 which controls the stream quantity of the molten steel from the above-described tundish 11 is provided at the molten steel entrance port 2 of the above-described nozzle 1 at the upper end, as shown in FIG. 4. Moreover, the lower part of the above-described nozzle is arranged in a mold 14 for the thin slab.

[0082] In the above-described device configuration the stopper head 12 is raised and the molten steel 13 is poured into the mold 14 from the tundish 11 through the molten steel stream channel 3 in the nozzle 1.

[0083] The above-described mold 14 for a thin slab usually has a flat shape and the dimension of the long side is equal to or larger than 9000 mm and that of the short side is equal to or less than 150 mm. The part of the nozzle 1 which is submerged in the mold 14 is formed according to the size of the above flat mold 14.

[0084] Accordingly, the molten steel discharged from the molten steel discharge opening of the nozzle generates a side stream F1 and a downward stream F2 in the mold in a well-balanced manner, as shown in FIG. 4.

[0085] Furthermore, generation of the splashes, which is caused by the side stream F1 at an early period of casting, is prevented by the spill port 6 on the bottom wall of the nozzle 1.

[0086] The above-described side stream F1 streams toward the sidewall of the mold 14 in the approximately horizontal direction and becomes an upstream after hitting the sidewall of the mold 14. Since inclusions in the molten steel may be floated and melted into the mold powder by the above upstream for removal, reduction in the number of defects of the cast metal may be realized.

[0087] On the other hand, the above-described downward stream F2 obliquely streams downward and becomes a slow downstream along the side wall of the relevant mold 14 after hitting the side wall of the mold 14.

[0088] Thus, use of the above-described nozzle may prevent the molten steel, which is discharged from the molten steel discharge openings 4, from generating an unstable molten steel stream such as the drift current in the mold 14.

[0089] Hereinafter, the present invention will be more specifically explained, referring to an embodiment. But the present invention is not limited to the following embodiment.

EXAMPLE 1

[0090] Continuous casting tests for 2100 tons of molten steel have been done, using the submerged nozzle for continuous thin-slab casting as shown in FIG. 1A and FIG. 1B, and a mold for a thin slab of 900 mm×80 mm.

[0091] The above submerged nozzle has a configuration in which the part 5 submerged in the molten steel is made of the alumina graphite material, the terminal point C of the taper at the long side in the intermediate section 3 b is located downward by 50 mm from the terminal below the terminal point C′ of the taper at the short side, and it is assumed that α=45°, β=60°, T/t=150%, and the port area of the spill port is 30% of the cross sectional area of the tubular part in the lower section 3 c of the nozzle.

[0092] Furthermore, in this submerged nozzle, the sectional area S_(3c) at the flat and straight tube section in the lower section 3 c of the molten steel stream channel is equal to the cross sectional area S_(3a) at the cylindrical section in the upper section 3 c, that is, S_(3a)=S_(3c).

[0093] The results of the above-described tests are shown in Table 1.

COMPARATIVE EXAMPLE 1

[0094] Continuous casting tests have been done in a similar manner to those of the example 1 with only one exception that a conventional submerged nozzle for continuous thin-slab casting, which is not provided with a spill port on the bottom wall of the nozzle as shown in FIG. 5A and FIG. 5B, is used.

[0095] The results of the above-described tests are shown in Table 1.

COMPARATIVE EXAMPLE 2

[0096] Continuous casting tests have been done in a similar manner to those of the example 1, using a submerged nozzle having a configuration which is similar to that of the example 1 and has only one exception that a part submerged in the molten steel is made of alumina graphite containing 22% silica by weight.

[0097] The results of the above-described tests are shown in Table 1.

COMPARATIVE EXAMPLE 3

[0098] Continuous casting tests were started in a similar manner to those of the example 1, using a submerged nozzle having a configuration which is similar to that of the example 1 and has only one exception that the terminal point C of the taper at the long side in the intermediate section 3 b is located at the same height as that of the terminal point C′ of the taper at the short side. But, the casting was stopped after 100 minutes had passed from the starting of the casting, as cracks were generated in parts at the above terminal points C and C′ of the above-described taper. TABLE 1 Comparative Comparative Example 1 example 1 example 2 Thickness of extraneous- 1.3 1.4 25.4 matter layer at lower section (mm) Fluctuation of molten steel Non-existing Non-existing Existing surface in mold Generation of slag beard Non-existing Non-existing Existing Steel-wire-type crack 0.0 0.0 23.3 incidence Steel blowhole incidence (%) 0.0 0.0 82.9 Breakout incidence (%) 0 5 Undetected Finish time for casting (min.) 540 540 380

[0099] As shown in Table 1 there have been found no generation of breakout due to the splashes in the nozzle with the spill port (example 1), but the breakout due to the splashes has occurred every twenty wires in the conventional nozzle (comparative example 1) in which the discharge openings are provided in only two directions.

[0100] In addition, it has been noted in the example 1 that the nozzle may be stably used for a long time, as the quantity of the adhesion of the ground metal to the circumference of the lower section is more suppressed in comparison with that of the case (comparative example 1) in which silica is included in the part which is submerged in the molten steel.

[0101] Especially, it has been found in the example 1 that the generation of the slag beard is completely suppressed as the surface fluctuation of the molten steel is little at the last period of the long time use by reduction in the extraneous matters and the uniform molten steel layer of the mold powder, which exists on the mold surface, is realized.

COMPARATIVE EXAMPLE 4

[0102] Continuous casting tests have been done in a similar manner to those of the example 1, using a submerged nozzle having a configuration which is similar to that of the example 1 and has only one exception that the cross sectional area S_(3c) at the flat and straight tube section in the lower section 3 c of the molten steel stream channel is 90% of the cross sectional area S_(3a) at the cylindrical section in the upper section 3 a.

[0103] In the above tests, the surface fluctuation of the molten steel in the mold and defects of the entrainment of the mold powder into the cast strand have been evaluated.

[0104] These results are shown in Table 2 with those of the example 1.

COMPARATIVE EXAMPLE 5

[0105] Continuous casting tests have been done in a similar manner to those of the example 1, using a submerged nozzle having a configuration which is similar to that of the example 1 and has only one exception that the cross sectional area S_(3c) at the flat and straight tube section in the lower section 3 c of the molten steel stream channel is 110% of the cross sectional area S_(3a) at the cylindrical section in the upper section 3 a.

[0106] In the above tests, the surface fluctuation of the molten steel in the mold and defects of the entrainment of the mold powder into the cast strand have been evaluated.

[0107] These results are shown in Table 2. TABLE 2 Comparative Comparative Example 1 example 4 example 5 S_(3c)/S_(3a) 100% 90% 110% Surface fluctuation of Non-existing Large Small molten steel in mold Defects of entrainment of Non-existing Less Many mold powder

[0108] As shown in Table 2, less defects of the entrainment of the mold powder are found, but surface fluctuation of the molten steel in the mold is large when S_(3c) is less than 95% of S_(3a) (comparative example 4), and, on the other hand, the surface fluctuation of the molten steel in the mold is small, but many defects of the entrainment of the mold powder are found when S_(3c) exceeds 105% of S_(3a) (comparative example 5). Preferable results have not been obtained as the quality of the cast strand in any nozzles.

[0109] On the other hand, when S_(3c) is equal to or larger than 95% or equal to or less than 105% of S_(3a) (example 1), there have been neither surface fluctuation of the molten steel in the mold, nor detected defects of the entrainment of the mold powder. Thereby, it has been found that the nozzle according to the present invention is effective for obtaining a high quality cast strand.

[0110] As described above, the submerged nozzle for continuous thin-slab casting according to the present invention may prevent a drift current and surface fluctuation of the molten steel in the nozzle and the mold, and may suppress the adhesion of the ground metal to the nozzle, the damages of the nozzle and the like even under long time use.

[0111] Accordingly, a high quality thin-slab strand may be stably supplied for a long time in a continuous manner by using the submerged nozzle for continuous thin-slab casting according to the present invention. 

What claimed is:
 1. A submerged nozzle for continuous thin-slab casting, which comprises: a molten steel entrance port at the upper end; a molten steel stream channel with a shape of a tube extending downward from the molten steel entrance port; and a molten steel discharge opening at the lower end and in which the molten steel stream channel includes an upper section with a shape of a circular cylinder, an intermediate section with a transforming shape from a shape of a circular cylinder to a shape of a flat cylinder, and a lower section with a shape of a flat and straight tube, wherein a cross sectional area at the flat and straight tube section in the lower section is equal to or larger than 95% or equal to or less than 105% of a cross sectional area at the cylindrical section in the upper section of the molten steel stream channel.
 2. A submerged nozzle for continuous thin-slab casting, which comprises: a molten steel entrance port at the upper end; a molten steel stream channel with a shape of a tube extending downward from the molten steel entrance port; and a molten steel discharge opening at the lower end and in which the molten steel stream channel includes an upper section with a shape of a circular cylinder, an intermediate section with a transforming shape from a shape of a circular cylinder to a shape of a flat cylinder, and a lower section with a shape of a flat and straight tube, wherein the intermediate section of the molten steel stream channel is provided with a taper shape thickened toward the end at the long side and a taper shape diminished toward the end at the short side and has a configuration in which the position of the starting point of the taper at the long side and that of the starting point of the taper at the short side, and that of the terminal point of the taper at the long side and that of the terminal point of the taper at the short side are different from each other with regard to their heights, a spill port is provided at the bottom center in the bottom wall of the nozzle, and the port area of the spill port is within the range of equal to or larger than 20% through equal to or less than 40% of the cross sectional area at the straight tube section in the lower section of the molten steel stream channel.
 3. The submerged nozzle for continuous thin-slab casting according to claim 2, wherein the cross sectional area at the flat and straight tube section in the lower section is equal to or larger than 95% or equal to or less than 105% of the cross sectional area at the cylindrical section in the upper section of the molten steel stream channel.
 4. The submerged nozzle for continuous thin-slab casting according to claim 1, wherein at least a section which is submerged in molten steel is formed of a fire resistant material which does not contain silica glass.
 5. The submerged nozzle for continuous thin-slab casting according to claim 2, wherein at least a section which is submerged in molten steel is formed of a fire resistant material which does not contain silica glass.
 6. The submerged nozzle for continuous thin-slab casting according to any one of claims 1, wherein the position of the terminal point of the taper at the long side and that of the terminal point of the taper at the short side are different from each other by equal to or larger than 10 mm with regard to their heights in the intermediate section of the molten steel stream channel.
 7. The submerged nozzle for continuous thin-slab casting according to claim 2, wherein the position of the terminal point of the taper at the long side and that of the terminal point of the taper at the short side are different from each other by equal to or larger than 10 mm with regard to their heights in the intermediate section of the molten steel stream channel.
 8. The submerged nozzle for continuous thin-slab casting according to any one of claims 1, wherein the molten steel discharge opening is formed between the side wall of the nozzle lower end at the short side and the side part of the plane-like bottom wall, and an angle α between the side wall of the nozzle at the lower end, which forms the molten steel discharge opening and the horizontal plane, and an angle β between the side part of the bottom wall, which forms the molten steel discharge opening and the horizontal plane are as follows: 0°≦α≦60°30°≦β≦80°.
 9. The submerged nozzle for continuous thin-slab casting according to claim 2, wherein the molten steel discharge opening is formed between the side wall of the nozzle lower end at the short side and the side part of the plane-like bottom wall, and an angle α between the side wall of the nozzle at the lower end, which forms the molten steel discharge opening and the horizontal plane, and an angle β between the side part of the bottom wall, which forms the molten steel discharge opening and the horizontal plane are as follows: 0°≦α≦60°30°≦β≦80°.
 10. The submerged nozzle for continuous thin-slab casting according to any one of claims 8, wherein a notch in the direction of the long side at the bottom wall of the nozzle is between equal to or larger than 75% and equal to or less than 200% of the thickness of the sidewall at the short side in length.
 11. The submerged nozzle for continuous thin-slab casting according to claim 9, wherein a notch in the direction of the long side at the bottom wall of the nozzle is between equal to or larger than 75% and equal to or less than 200% of the thickness of the sidewall at the short side in length. 